ABB REL670 2.2 IEC Applications Manual page 340

Section 8
Impedance protection
334
Where:
U is the highest fundamental frequency voltage on one of the healthy phases at single
max
phase-to-earth fault.
U is the phase-to-earth fundamental frequency voltage before fault.
pn
Another definition for effectively earthed network is when the following
relationships between the symmetrical components of the network impedances are
valid, see equation 288
< ×
X 3 X
0 1
EQUATION2122 V1 EN-US
£
R
R
0 1
EQUATION2123 V1 EN-US
Where
R is the resistive zero sequence of the source
0
X is the reactive zero sequence of the source
0
R is the resistive positive sequence of the source
1
X is the reactive positive sequence of the source
1
The magnitude of the earth-fault current in effectively earthed networks is high
enough for impedance measuring elements to detect earth faults. However, in the
same way as for solidly earthed networks, distance protection has limited
possibilities to detect high resistance faults and should therefore always be
complemented with other protection function(s) that can carry out the fault
clearance in this case.
High impedance earthed networks
In high impedance networks, the neutral of the system transformers are connected
to the earth through high impedance, mostly a reactance in parallel with a high
resistor.
This type of network is often operated radially, but can also be found operating as a
meshed network.
What is typical for this type of network is that the magnitude of the earth -fault
current is very low compared to the short circuit current. The voltage on the
healthy phases will get a magnitude of √3 times the phase voltage during the fault.
The zero sequence voltage (3U
in the network due to low voltage drop distribution.
The magnitude of the total fault current can be calculated according to
equation290.
and 289.
) will have the same magnitude in different places
0
1MRK 506 369-UEN B
(Equation 288)
(Equation 289)
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Line distance protection REL670 2.2 IEC
Application manual